1. Field of the Invention
This invention relates to direct flame impingement heating applications. More particularly, this invention relates to a method and apparatus for heating ferrous and non-ferrous objects in which the flame resulting from combustion of a fuel/oxidant mixture is allowed to impinge directly upon the surface of the objects to be heated, by virtue of which the bulk temperature of the product is reached more efficiently, over a shorter period of time, with reduced scale loss and lower undesirable emissions than with conventional methods and apparatuses employed for heating such objects.
2. Description of Related Art
Many industrial processes require rapid heating of metals, which is typically accomplished by using gas-fired furnaces that are designed to increase radiation heat transfer or electric induction furnaces. For many applications, however, a more attractive alternative is to use gas-fired furnaces that are designed to increase convective heat transfer. In recent years, interest in applying the concept of direct flame impingement in industrial furnaces to enhance convective heat transfer rates has increased. In some applications, direct flame impingement is emerging as an attractive and cost-effective alternative to conventional radiant heating. Use of direct flame impingement offers several potential advantages over radiant heating, such as increased heat fluxes, which reduces processing time, fuel consumption and undesirable scale formation (oxidation), and which improves product quality. In addition, with multi-flame direct flame impingement, the heating can be locally targeted by adjusting the firing rate of the individual flames.
Applications for direct flame impingement include continuous heating of tubes and strips and reheating of billets and slabs. In conventional gas-fired furnaces used for these processes, the convective heat transfer coefficient is usually less than 50 W/m2 K (9.0 Btu/ft2h-F). With direct flame impingement, however, this coefficient can be increased to several times this value. During the past several years, a number of rapid heating technologies using different jet impingement approaches have been investigated. Primary differences between these approaches are in the type of jet used and their makeup. In one approach, high velocity combustion products are generated by special tunnel burners and little or no combustibles are present in the impingement zone. An alternative approach utilizes high velocity flames with intense fuel/oxidant mixing and burning in the impingement zone.
The use of rapid heating technologies based on impinging jets of hot combustion products has grown steadily over the past decade in the metallurgical industry. This is primarily due to the development of burners having high exit velocities. For many applications, a more effective alternative for rapid heating, however, might be to apply direct impingement of multiple flames. This approach involves installation of several steel nozzles, almost flush with the internal surfaces of the furnace walls, to direct high-velocity premixed fuel/air mixture flames at the product surface. No separate combustion chambers or tunnels or flame holders are used. To ensure very high heat transfer rates, velocities to Mach 1 are employed. Although the approach is simple, unlike conventional furnaces equipped with tunnel burners, the high velocity multi-flame direct flame impingement system requires more critical furnace design as well as accurate definitions of key parameters, including nozzle diameters and their spacing, distance between the nozzles and the metal product, and the firing rate per-unit area of the metal product surface. Improper specification of these parameters can result in unstable and incomplete combustion as well as reduced thermal efficiency.